1. Field of the Invention
The present invention relates to the field of superconducting devices which are utilized as, e.g., switching elements for computers adapted to perform operations, storage, etc. at high speed. More particularly, it relates to an oxide-superconducting device which can operate at or above the liquid nitrogen temperature.
2. Description of the Related Art
A superconducting current coupling device in a prior art has a structure wherein a superconductor film for a base electrode, a tunnel barrier layer, and a superconductor film for a counter electrode are successively stacked. The tunnel barrier layer has usually been formed by oxidizing the surface layer of the base electrode film, by depositing a very thin layer of semiconductor or insulator film, or by depositing and then oxidizing a very thin layer of metal.
By way of example, in case of a superconducting current coupling device which employs Nb as its superconductor, an Al thin layer having a thickness of 3-10 nm is deposited en an Nb film as a base electrode and is oxidized, thereby to form a tunnel barrier layer. The structure and manufacturing method of such an Nb-type superconducting current coupling device are described in detail in "IEEE Transactions on Magnetics," MAG-19 (1983), pp. 791-794. Such an Nb-type superconducting current coupling device operates at, at most, 9 K being the critical temperature of Nb. In order to operate such an Nb-type device, cooling with liquid helium is required, and the equipment and cost of the cooling become enormous. In contrast, if a device capable of operating at a high temperature such as the liquid nitrogen temperature exists, it will be very advantageous from the viewpoints of the cooling cost and easy handling.
For fabricating a superconducting current coupling device of high operating temperature relative to the prior art stated above, a superconductor material of high critical temperature needs to be employed for electrode films. As the superconductor material of high critical temperature, an oxide having the perovskite type crystal structure, such as Ba--Y--Cu oxide, or an oxide having the modified structure thereof, for example, La--Ba--Cu oxide or Bi--Sr--Ca--Cu oxide, exhibits a high superconducting critical temperature of or above 90 K. When the Ba--Y--Cu oxide is used as the electrode material, the superconducting current coupling device which operates at a temperature up to 90 K is fabricated.
A thin film is employed as the electrode member of the superconducting current coupling device. A temperature for forming the thin film of the Ba--Y--Cu oxide is above 800.degree. C. Assuming the electrode members of the Ba--Y--Cu oxide and a tunnel barrier layer made of an insulator or semiconductor and having a thickness of about 2 nm, let's consider the junction structure of the prior-art system wherein these layers are stacked vertically to a substrate. As the steps of fabrication for obtaining such a superconducting current coupling device adapted to operate at the high temperature, it is necessary to form the thin film of the Ba--Y--Cu oxide as the base electrode, to thereafter cover this thin film with the insulator or semiconductor to the thickness of about 2 nm, and to further form the thin film of the Ba--Y--Cu oxide as the counter electrode. In a case where the substrate is heated above 800.degree. C. at the step of forming the Ba--Y--Cu oxide for the counter electrode, the semiconductor or insulator being 2 nm thick is destroyed by thermal diffusion. More specifically, it is impossible for a typical insulating ultrathin film of SiO.sub.2, Al.sub.2 O.sub.3 or the like to be free from pinholes and hold its insulating property even after the heat treatment at above 800.degree. C. The reason is that, in the heating process, a diffusion reaction takes place between the Ba--Y--Cu oxide and the insulating oxide. It has accordingly been impossible to apply the prior-art method to the fabrication of the superconducting current coupling device of high temperature operation which employs the superconductor material of high critical temperature such as Ba--Y--Cu oxide.